In many applications today it is desirable to use an audio amplifier to drive multiple loudspeakers. For example, home systems, auditoriums, sports arenas, concert halls and theaters are some of the situations in which multiple speakers are used.
Another application in which it would be advantageous to connect multiple speakers to a single amplifier is at an outdoor concert. People are often far away from the stage and would be unable to hear the music without additional speakers located throughout the venue. One way in which to connect all of these speakers to a single amplifier channel is to connect them in parallel. The speakers can be connected to an amplifier output in this manner as long as their combined load impedance does not fall beneath the amplifier's required minimum load impedance. However, an amplifier's minimum load impedance is typically 4 or 8 ohms and modern speakers are also commonly made in a 4 or an 8 ohm version which means that a very limited number of speakers can be connected in parallel which would not be useful for an outdoor concert.
Another way in which to connect multiple speakers for an outdoor concert would be to place a high-power resistor (a 3-Ohm 50 W resister is used as an example) in series with the amplifier output and then attach parallel speakers. However, this type of connection is very inefficient; most of the amplifier power goes towards heating the high-power series resistor instead of powering the speakers. This method also would not be advantageous for an outdoor concert because only a small number of speakers could be connected in this manner.
It is desirable to be able to individually control the volume of each of the speakers connected to a single audio amplifier channel and to be able to turn each speaker on and off independently. For example, homes often have speakers located in multiple rooms throughout the house that are driven and controlled from a central location using a single audio amplifier. The volume level of the speakers in one room may need to be set at a different level than the volume of the speakers in another room, or even speakers in the same room may need to be at different volumes. For example, if a person is having a party, the volume of the speakers located in the dining room could be set at a lower volume level than the speakers on the patio. Whereas, later in the evening the speakers on the patio may need to be turned down or even off to avoid upsetting the neighbors.
One way in which to connect multiple speakers to a single amplifier that allows for individual control of the speakers is by impedance matching through the use of variable autoformers (commonly called a volume control in the audio industry). By properly selecting the turns ratio of the autoformers, the attached speakers can present an optimal reflected impedance to the amplifier. However, to insure the impedance presented to the amplifier does not drop below the minimum impedance of the amplifier, the turns ratio of the autoformers must always be set for a worst case scenario. For example, if eight, 8 ohm loudspeakers are connected to an amplifier channel through autoformers, then each speaker can only access ⅛ of the amplifier channel output power regardless of how many of the speakers are turned on. This means that the amount of power that is used to drive each speaker cannot be individually controlled. For example, if the speakers in one room of a house are turned off, then the power that was being used to drive them cannot be reallocated to the remaining speakers.
The above issues as well as others have presented challenges to connecting multiple loudspeakers to a single audio amplifier channel for a variety of applications.